The present invention relates to drilling tools and more particularly to helically fluted twist drills larger than approximately 0.125 inches in diameter.
Helically fluted twist drills are the most commonly used drilling tools and are required to perform severe machining operations under extremely adverse conditions. The cutting end of a helically fluted twist drill includes a pair of cutting lips on opposite surfaces of an intermediate web, the width of which is typically 12 to 20% of the diameter, and a chisel edge extending obliquely across the center of the web. Such drills are typically long and slender and the helical flutes constitute a column eccentricity that further reduces rigidity under axial thrust load.
The concept of oppositely directed cutting surfaces at one extremity of a slender shaft which is both axially and torsionally loaded creates conflicting material demands in the construction of the drilling tool. The material of the cutting lips should be as hard as possible to cut the workpiece and as heatresistant as possible to maintain a cutting edge at elevated temperatures. At the same time the material of the body and shaft must be both rigid and tough to resist deflection and to hold up under the loadings imposed. These varying requirements have resulted in compromises in material selection, since hard materials tend to be brittle, while tough materials tend to wear easily.
To obtain an optimum combination of characteristics, i.e., hardness and wear-resistance at the cutting surfaces and toughness and rigidity of the body and shaft, it has been proposed to form the cutting surfaces of one material and the body and shaft of another. This has resulted in a variety of combinations, such as tungsten carbide or diamond inserts or tips on carbon steel or carbide shafts. These combinations, while individually useful, have a common disadvantage, i.e., the braze connection between the insert or tip and the shaft. Tungsten carbide can be soldered or brazed directly to the steel or carbide shaft. However, a diamond tip or insert must first be adhered to a carbide substrate which is in turn soldered or brazed to the shaft. Diamond particles are typically formed into a compact and bonded to a carbide substrate with a metallic catalyst in a high pressure/high temperature (HP/HT) press. However, at atmospheric pressure the metal which catalyzes the bonding of the diamond particles to each other and to the substrate in the press will also catalyze the back-conversion of diamond to graphite at temperatures above 700.degree. C., causing disintegration of the compact. Accordingly, a low temperature solder or braze connection is used to attach the substrate to the shaft. This braze connection limits the effective life of such drilling tools, since it is softer than either the substrate or the shaft. The braze thus becomes the weakest point of the tool construction and the limiting factor in the tool usage.
In co-pending applications Ser. No. 515,777 and Ser. No. 793,202, a process is disclosed for depositing a vein of diamond particles in a groove in one extremity of a cemented carbide shaft. With this process the particles are bonded directly to each other and directly to the carbide material of the shaft, such that the connection between the particles and the carbide becomes the strongest part of the drilling tool. The process as disclosed has particular applicability to printed circuit board drills which have diameters of approximately 0.006 to 0.125 inches and in which the vein occupies the full width of the web which may be from 0.0012 to 0.030 inches wide. However, the process has not been applicable to large drills since cracking of the particle mass of the vein is encountered at vein widths of approximately 0.030 inches and above.